Lunar Straight Wall (Chris Vaughan, Starry Night Education)

This Week’s Sky: January 18 to 24, 2021

Check out lunar features this week as the Moon waxes to gibbous.

Wednesday, January 20 – First quarter Moon (at 21:01 GMT)

First quarter Moon (Chris Vaughan, Starry Night Education)

When the Moon completes the first quarter of its orbit around Earth at 4:01 p.m. EST (or 21:01 GMT) on Wednesday, January 20, the relative positions of the Earth, sun, and Moon will cause us to see it half-illuminated – on its eastern side. At first quarter, the Moon always rises around noon and sets around midnight, so it is also visible in the afternoon daytime sky. The evenings surrounding first quarter are the best for seeing the lunar terrain when it is dramatically lit by low-angled sunlight.

Wednesday, January 20 – Moon, Mars and Uranus (evening)

Moon, Mars and Uranus (Chris Vaughan, Starry Night Education)

In the southwestern sky on the evening of Wednesday, January 20, the waxing, half-illuminated Moon will pass Mars and Uranus, which will be reaching their minimum separation that night. After dusk the Moon will be positioned a palm’s width below (or 7 degrees to the celestial southwest of) bright Mars, with much dimmer Uranus sitting a thumb’s width below (or 1.6 degrees south of) the reddish planet. By the time Mars sets in the west after midnight local time, the diurnal rotation of the sky will slide the Moon to the planets’ lower left.

Thursday, January 21 – The Lunar Straight Wall (evening)

Lunar Straight Wall (Chris Vaughan, Starry Night Education)

On Monday evening, January 21, the pole-to-pole terminator boundary that divides the lit and dark hemispheres of the waxing gibbous Moon, will fall just to the left (or lunar west) of Rupes Recta, also known as the Lunar Straight Wall. This feature is very obvious in good binoculars and backyard telescopes. The rupes, Latin for “cliff,” is a north-south aligned fault scarp that extends for 65 miles (110 km) across the southeastern part of Mare Nubium — that’s the large dark region in the lower third of the Moon’s Earth-facing hemisphere. The Straight Wall is always prominent a day or two after first quarter, and again just before third quarter. For reference, the prominent crater Tycho is located due south of the Straight Wall. 

Saturday, January 23 – Vesta stands still (overnight)

Vesta stands still (Chris Vaughan, Starry Night Education)

On Saturday, January 23, the large main belt asteroid designated (4) Vesta will begin a westward retrograde loop (red path with dates) through the stars of Leo. The loop will continue through its April 4 opposition, and into late April. Tonight, magnitude 6.65 Vesta can be found sitting several finger widths to the right (or 4.25 degrees to the celestial south) of the bright star Denebola, the lion’s tail. Vesta will cross the southern sky during the bulk of the night.

Saturday, January 23 – Mercury at Greatest Eastern Elongation (evening)

Mercury at Greatest Eastern Elongation (Chris Vaughan, Starry Night Education)

In the west-southwestern sky on the evening of Saturday, January 23, Mercury (orbit shown in red) will reach its widest separation, 19 degrees east of the Sun. With Mercury positioned close to the evening ecliptic (green line), this appearance of the planet will offer excellent views for Northern Hemisphere observers, but it will not be ideal for observers in the Southern Hemisphere. The optimal viewing times at mid-northern latitudes fall around 6 p.m. local time. Viewed in a telescope (inset) the planet will exhibit a waning, half-illuminated phase.

Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the universe together.